A Brief History of Deaths from Tornadoes in the United States

Submitted for consideration in a Special Issue of Weather and Forecasting
on
the 3 May 1999 Tornado Outbreak

January 2001

Corresponding Author: Harold E. Brooks, NOAA/NSSL,
Norman, OK 73069.

Abstract

The 3 May 1999 Oklahoma City tornado was
the deadliest in over 20 years, with 36 direct fatalities. In order to
understand how this event fits into the historical context, we have examined
the record of tornado deaths in the United States. Almost 20,000 deaths
have been reported associated with more than 3600 tornadoes in the United
States since 1680. A cursory examination of the record shows a break in
1875. Prior to then, it is likely that many killer tornadoes failed to
be reported. When the death toll is normalized by population, a near constant
rate of death is apparent until about 1925, when a sharp fall begins. The
rate was about 1.8 people per million population in 1925, and was less
than 0.12 people per million by 2000. The decrease in fatalities has resulted
from two primary causes: a decrease in the number of killer tornadoes and
a decrease in the number of fatalities in the most deadly tornadoes. Current
death rates for mobile home residents, however, are still nearly what the
overall national rate was prior to 1925 and are about 20 times the rate
of site-built home residents. The increase in the fraction of the U.S.
population living in mobile homes has important implications for future
reductions in the death toll.

1. Introduction

Tornadoes are a hazard to life and property in the
United States. Recently, an analysis of historical changes in damage associated
with major tornadoes has shown that, if the damage amounts are adjusted
for wealth of the country, there is no clear long-term trend in the damage
caused by the most damaging tornadoes (Brooks and Doswell 2001). In this
follow-up work, we examine the question of changes in the record of tornado
deaths in the United States, focusing primarily on the annual number of
deaths, killer tornadoes, and fatalities per tornado. As is the case with
the damage dataset, there are issues involved in the use of the data that
must be addressed. After discussing those issues, we will look at the long-term
record in an effort to determine changes through time. We will close by
considering possible future trends in tornado fatalities.

These considerations provide a background for understanding
the fatalities in the 3 May 1999 outbreak. The tornado that hit the metropolitan
Oklahoma City area killed 36 people, the most in the U.S. since the 10
April 1979 Wichita Falls, Texas tornado killed 42 (see Table
1 for a record of the most fatalities in a single U.S. tornado going
backwards in time from the end of 2000). The total number of fatalities
in the outbreak was 46, the most in the U.S. since the Ohio-Pennsylvania
outbreak of 31 May 1985 killed 76. Of the 36 fatalities in the Oklahoma
City tornado, 11 occurred in mobile homes, and 18 of the 46 deaths in the
outbreak were mobile home residents. As we will discuss later, the problem
of mobile home safety is the biggest obstacle to reduction of tornado deaths
in the U.S.

2. The dataset and concerns

The primary dataset to be used is that of Grazulis (1993,
hereafter G93; 1995), listing tornadoes with fatalities up through 1995.
From 1996-2000, the records are those collected by NOAA's Storm Prediction
Center (SPC, 2000) According to G93, the first death from a tornado in
the United States was recorded in 1680 in Cambridge, Massachusetts. Tornado
fatalities were reported infrequently over the following two centuries,
and tornadoes with at least ten fatalities were reported very rarely. From
1680-1874, only four double-digit death tornadoes were recorded. In contrast,
seven were recorded in the decade that followed. As a result, the primary
emphasis of this paper is on the period 1875-2000. Along with the apparent
break in the number of tornadoes with at least ten fatalities, another
reason for concentrating on the period beginning in 1875 is clear when
the number of killer tornadoes per year is considered (Fig.
1). In 1875, a large increase in the number of reports in the record
occurred, with 30 killer tornadoes in that year alone.

It seems plausible, given the abrupt change in the number
of killer tornadoes, that many killer tornadoes prior to 1875 went unreported.
Given the rapid change in demographics in the U.S. in the 19th century,
it is extremely difficult to estimate the amount of underreporting. Even
since then, it is likely that some killer tornadoes have gone unreported,
although underreporting is probably less of a problem since 1875.

The number of deaths in individual tornadoes is not always
well-known, particularly for earlier tornadoes. G93 reports that possible
deaths of slaves in the 1840 Natchez, Mississippi tornado are not recorded,
as well as deaths on boats in the Mississippi River in the 1896 Saint Louis-East
Saint Louis tornado. The latter event also provides evidence of conflicting
numbers of deaths from different sources. G93 lists the number of deaths
in Saint Louis as 137 and East Saint Louis as 118, consistent with Curzon
(1896), who published a book on the tornado within two weeks of the event.
The East Saint Louis Journal newspaper (Cook and Reed 2001) listed 128
deaths in East Saint Louis 4 days after the tornado, and Williams (1999)
lists 140 dead in Saint Louis, giving a total of 268. We will use the G93
numbers throughout, with the caution that the exact number of deaths in
many events cannot be known.

Another problem involves determining precisely which
deaths are tornado-related. In the case of the 3 May 1999 Oklahoma City
tornado, 36 people were killed directly by the tornado (e.g., hit by debris
or thrown long distances), but several others could be viewed as indirect
deaths. For instance, at least two people died of heart attacks in the
vicinity of the damage path, a third died in a fall trying to get into
a storm shelter outside of the tornado, and another died in a fire when
candles, lit after his electrical power had gone off because of the tornado,
caught his residence on fire (P. Archer, personal communication, 2000).
Traditionally, National Weather Service procedure has been to consider
only the direct fatalities, but determining that information, particularly
early in the record, is impossible. There is no way to know how many of
the deaths within the record are indirect, so there is a possible problem
of inhomogeneity in the dataset.

3. General features of the record

The number of deaths per year in tornadoes has generally
been much less in the last quarter of the 20th century than it was previously
(Fig. 2). In particular, the number of deaths in the
"big years" has dropped dramatically. From 1975-2000, only twice did 100
tornado deaths occur in a year (1984 and 1998). In contrast, it occurred
54 times from 1875-1974, including every year from 1916 through 1927. The
1910s through the middle 1930s, in fact, represent a period of very high
fatalities, as shown by the smoothed curve on Fig. 2.
From 1912-1936, the mean annual death toll was 260, almost five times as
many as in 1976-2000, when the mean was 54.

The decrease in death toll is even more apparent when
the rate of death is normalized by the population of the United States,
available from the United States Census Bureau1
(Fig. 3). Prior to 1925, the smoothed rate was relatively
constant, showing a slight increase from about 1.6 per million to 1.8 per
million. Since that time, however, the rate has decreased substantially,
to a little over 0.11 per million, a factor of roughly 15 less than the
long-term average prior to 1925. Many factors, including the beginnings
of tornado forecasting, improved communications, spotter networks, and
changes in construction, likely have been involved in this decrease (see
Doswell et al. 1999 for more discussion). There is no simple way to deconvolve
the possible contributions from all those factors so that we do not assign
a relative importance to any of them in particular. To put the change into
perspective, however, a death rate of 1.8 per million people per year would
have resulted in approximately 500 deaths in the year 2000, compared to
the actual number of 40.

A long-term decrease in the rate of tornado deaths also
means that the notion of an "average" tornado death toll is someone difficult
to define. Taking a standard climatological period of 30 years makes little
sense, given that the presence of such a strong trend. A more reasonable
approach might be to look at the expected value of the trend at whatever
point on the time series is of interest. In 2000, with the value of 0.12
per million and a population of almost 280 million, the expected death
toll would be about 34, half of the 30-year mean value of 68. The decrease
in the death toll also has changed the perception of what is a "big" year
for tornado deaths. Lines estimating the 90th percentile death rate from
1925-2000 have been constructed to be parallel to the mean value (Fig.
3). Multiplying the values of the 90th percentile by the population
in 1925 and 2000 gives estimated values of 475 and 80 deaths per year at
those two times. From this perspective, the 1998 national death toll of
130 was just as far above the expected number of deaths as the 1925 national
death toll of 794 (most of which came from the single "Tri-State" tornado).
In fact, the 10th percentile value in 1925 is 61, so that a rare low death
toll 75 years ago is almost as big as a rare high death toll now.

Identifying the underlying social and scientific reasons
behind the decline in tornado deaths is difficult, but we can at least
attempt to identify how the decrease has occurred from a statistical perspective.
First, the number of killer tornadoes is, in general, less than it was
75 years ago (see Fig. 1). The median number of killer
tornadoes per year from 1976-2000 was 17 and the maximum was 33. In contrast,
the median from 1912-1936 was 39 and the minimum was 17.

Arguably more important than the number of killer tornadoes,
the number of deaths in individual "high-death" tornadoes has decreased.
This can be seen in at least two different ways. First, we can define a
"high-death" tornado and examine how that definition changes with time.
A simple, objective definition is to take all killer tornadoes within a
five-year window and find the 95th percentile of the death tolls in those
tornadoes (Fig. 4). Smoothing the 95th percentile value
with a 3-point median filter, followed by an 11-point running mean, shows
that it has dropped from 20-25 during the period 1900-1970 down to about
10-15 since 1975.

A second illustration of this is the return period of
any particular value of the number of deaths in a single tornado. As an
example, we have calculated how many days there were between two days with
at least 10-death tornado on them since 1875, a total of 259 days as a
function of time (Fig. 5). Prior to 1958, return periods
of less than 10 days were common. Since then, the fewest days between 10-death
tornado days has been 19 (between the Hall County, Georgia 12-fatality
tornado on 20 March 1998 and the Birmingham, Alabama 32-fatality tornado
on 8 April 1998).

We can compare the return period for 10-death tornadoes
to the population of the U.S. By smoothing the return periods with a strong
5-point median and 19-point running mean, a shortening of the return period
is apparent prior to 1925 with a lengthening of the period since then (Fig.
6). In the absence of mitigation efforts, it might be expected that
the return period would have an inverse relationship to population. During
the period before 1925, this is roughly true. Since then, however, as population
has continued to increase, the return period has increased. Again, we cannot
identify exactly why, but seems likely that improved forecasting, communications,
spotter networks, and construction has provided significant mitigation
of the life-threatening consequences of tornadoes.

4. The mobile home problem

Since 1975, the National Weather Service has separated
out tornado fatalities associated with mobile homes. Data for other fatality
locations (permanent2 homes,
other buildings, vehicles, outdoors) are available online from the Storm
Prediction Center (http://www.spc.noaa.gov/climo/)
starting with 1985 (Table 2). The fraction of deaths
occurring in mobile homes has been increasing since 1975 (Fig.
7). Using a Kendall's tau test (Press et al. 1992), the increase is
statistically significant at the 95% confidence level. From 1976-1980,
24% of tornado deaths were in mobile homes. That fraction increased to
34% in 1986-1990 and 50% from 1996-2000.

In order to understand the increase, we need to look
at the changing demographics of the U.S. The Census Bureau carries out
a housing survey with the decennial census to determine the number of different
kinds of housing units in use in the U.S.3
The mobile home fraction of housing units in the U.S. has increased significantly
since 1950 (Table 3). Assuming the number of people
in different kinds of housing is the same for each kind of unit, that means
that 10 times as many people lived in mobile homes in 1990 compared to
1950. We can use these data to calculate the rate of fatalities in each
kind of housing per million people, in a way similar to Fig. 3. For simplicity,
we have assumed that the fraction of housing units changed linearly between
the years the housing surveys were done and that the increase from 1980
to 1990 continued after 1990. The rate of death in mobile homes is dramatically
higher than in permanent homes (Fig. 8). The mean (median)
annual death rate from 1975-2000 in mobile homes was 1.23 (1.19) per million
per year, while the mean (median) rate from 1985-2000 in permanent housing
was 0.06 (0.04) per million per year. In short, the death rate is approximately
20 times as high in mobile homes compared to in permanent homes, and the
mobile home rate is about 70% of the rate for the total U.S. population
prior to 1925. There is no obvious trend in the rate of deaths over the
time period of record.

The effect of the increasing population in mobile homes
is important for the number of deaths in tornadoes nationally and may be
especially serious in the future. We can make a simple model of the effect
of mobile homes by using the population in mobile homes that underlies
Fig.
8 and by assuming that the average death rate in mobile homes occurred
each year since 1960. Further assuming that the mobile home population
will continue to increase as it has, we can estimate the number of deaths
in mobile homes in the future. Taking that number and dividing it by the
total U.S. population gives a death rate for the total U.S. population
if all deaths outside of mobile homes were eliminated. We can compare the
model death rate to the long-term trend seen in Fig. 3 (Fig.
9). The mobile home rate is approaching the long-term overall trend
and, in fact, will soon pass it. The raw data after 1990 in Fig. 3 could
be interpreted as showing that the long-term decline has stopped, and that
the death rate has leveled off, although it is too early to be sure of
this change. Thus, the increase in mobile home residency may have reversed
the approximately 70-year long trend of decreasing death rates in tornadoes.
Only time will tell what the trend will be or if efforts to target this
vulnerable population will help reduce deaths.

The increase in mobile homes as housing has been particularly
large in the southeastern U.S. (Fig. 10). Given that
this part of the U.S. gets a large number of tornadoes, and that many of
them happen overnight when getting messages to people is particularly difficult,
as shown by the tornadoes in Orlando, Florida (1998), Hall County, Georgia
(1998), and Camilla, Georgia (2000), this is an area of great concern.
Improvements in the accuracy and lead time of forecasts and warnings may
have little effect on decreasing deaths if the problem of mobile home deaths
cannot be addressed.

5. Summary

More than 18,000 people died in tornadoes in the U.S.
from 1875 to 2000. The rate of death has been decreasing since 1925 from
about 1.8 per million per year to 0.12 per million per year. This has resulted
from both fewer killer tornadoes and from a decrease in the number of deaths
in the largest fatality tornadoes. What used to be considered a relatively
common number of fatalities, both from the perspective of a single tornado
and annual death tolls, is now an extremely large number. We cannot determine
the importance of the various factors that have helped the decrease, so
that we cannot estimate the importance of forecasting or any other particular
activity.

There remains one particularly vulnerable group of people
in the U.S., residents of mobile homes. The rate of death is relatively
close to the pre-1925 values in the U.S. The increase in use of mobile
homes for housing has meant that they are an increasingly large component
of the overall death toll. Half of all fatalities from 1996 to 2000 occurred
in mobile homes, more than twice the fraction twenty years earlier. It
seems likely that the fraction of deaths in mobile homes will continue
to increase. Efforts to improve safety practices and communication of forecast
and warning information for this group will be especially important in
the future, if the long-term decrease in tornado fatalities in the U.S.
is going to continue.

Acknowledgments:

We thank Tom Grazulis for providing an electronic form
of his data base. Also, discussions and information from Debra Combs of
the United States Centers for Disease Control and Prevention and Pam Archer
of the Oklahoma State Department of Health were helpful, particularly in
illuminating the problem of indirect fatalities.

Table 1: Most fatalities in a single tornado
in the U.S. going back in time from the end of 2000. Location is most prominent
site or sites hit by tornado.

Year

Total

Mobile

Permanent

1975

60

13

1976

44

18

1977

43

7

1978

53

15

1979

84

9

1980

28

11

1981

24

14

1982

64

20

1983

34

20

1984

122

43

1985

94

28

40

1986

15

7

3

1987

59

24

7

1988

32

21

6

1989

48

12

8

1990

53

7

11

1991

39

20

3

1992

39

20

18

1993

33

13

6

1994

68

26

14

1995

30

8

15

1996

25

14

8

1997

67

30

23

1998

130

65

40

1999

93

39

35

2000

40

31

4

Table 2: Annual tornado deaths by selected
locations in U.S., 1975-2000. Total is the total number of deaths, Mobile
is number of mobile home deaths, and Permanent is number of permanent ("site-built")
housing deaths.

Mobile

Permanent

1950

0.7

99.3

1960

1.3

98.7

1970

3.1

96.9

1980

5.1

94.9

1990

7.2

92.8

Table 3: Percentage of housing units in U.S.
by mobile and permanent housing from U.S. Census Bureau decennial housing
survey.

Fig.
2: Deaths per year in tornadoes in United States, 1875-2000. Light
line with dots shows raw values. Heavy line is smoothed values. Smoother
consists of finding median of 3 values and then applying a running mean
over 5 medians.

Fig.
3: Death rate per million people per year in U.S., 1875-2000. Thin
line with dots is raw rate, curved thick line is death rate, filtered by
3-point median and 5-point running mean filter, and straight solid lines
are least squares fit to filtered death rate for 1875-1925 and 1925-2000.
Dashed lines are estimates of 10th and 90th percentile death rates from
1925-2000.

Fig.
4: Deaths in 95th percentile tornado. Thin line with dots is raw value
for 5-year period, centered on plotted year. Heavy line is filtered, using
a 3-point median and 11-year running mean.

Fig.
5: Days between days with at least one 10-fatality tornado (return
period). Dots are plotted at second date of pair.

Fig.
7: Fraction of total deaths occurring in mobile homes, 1975-2000. Thin
line with solid dots is for individual years and thick line with open dots
is for 5-year average with points plotted at center of period.

Fig.
8: Death rate per million people per year in U.S. for mobile home residents
(1975-2000, solid dots) and permanent home residents (1985-2000, open dots).

Fig.
9: Long-term trend in tornado death rate in U.S., based on 1925-2000,
extrapolated out to 2020 (solid line) and death rate from model based on
mean annual death rate in mobile homes, assuming no deaths occur except
in mobile homes and that historical increase in fraction of U.S. population
living in mobile homes will continue (dashed line).

Fig.
10: Fraction of total housing units that are mobile homes, based on
U.S. Census decennial survey of housing. Southeastern states, including
Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, Tennessee,
North Carolina, and South Carolina (solid line with open dots), central
Plains states, including Oklahoma, Kansas, and Nebraska (dashed line with
triangles), and rest of the U.S. (solid line with solid dots).

1.All population
figures in this paper were taken from United States Census Bureau web site
(http://www.census.gov/). The population figures used for Fig. 3 were the
July
1 annual estimates from 1900-1999 (http://www.census.gov/population/estimates/nation/popclockest.txt).
Prior to 1900, population was assumed to increase linearly between the
decennial censuses. For the year 2000, the change from 1998 to 1999 was
added to the 1999 figures.

2.Definitions
are not universally accepted. "Permanent" homes are often referred to as
"site-built" and "Mobile homes" are often referred to as "manufactured
housing."

3.The results
of the 2000 Housing Survey will become available in December 2001.